Cleanup QML imports and license headers

- Group the imports into Qt, KDE, NeoChat
- Import the latest versions
- Remove unused imports
- Remove unused components
- Unify license header styling

imports/NeoChat/Component/ChatBox/CompletionMenu.qml

@@ -5,7 +5,8 @@
 import QtQuick 2.15
 import QtQuick.Layouts 1.15
 import QtQuick.Controls 2.15
-import org.kde.kirigami 2.14 as Kirigami
+
+import org.kde.kirigami 2.15 as Kirigami
 
 import org.kde.neochat 1.0